But let’s say Michael Phelps couldn’t resist the urge to show the world he’s the winningest Olympian ever by wearing all his gold all the time. Would his body be able to withstand all that weight around his neck? —Max Cohen, The Wall Street Journal, 11 Aug. 2016

While 'winningest' has been in fairly consistent use since the middle of the 17th century, it has mostly meant that one was the most winning in the sense of “pleasing or attractive to other people" rather than “victorious.”

This word, the superlative form of winning, was thought to have been a recent addition to the English language, but recent findings have shown that it has been in use for a considerable length of time.

In like manner, although nothing should be more welcome to mankinde, (because nothing more necessary) than the news of a Saviour for sinners … though applyed to them in the best and winningest matter, that humane abilities can attaine: Nature and Reason teach there is a God, and no miracle was ever wrought to convert Atheists.… John Gauden, A Defence by Way of Apology for the Ministry and Ministers of the Church of England, 1653

Although winningest has been in fairly consistent use since the middle of the 17th century, it has mostly carried a slightly different meaning than the one applied to Phelps, indicating that someone was the most winning in the sense of “pleasing or attractive to other people” rather than “victorious.”

The word has been in use to refer to a particularly successful entity in sports since at least 1933, when an article in The Chicago Daily Tribune referred to the Mineralites and the Illinois Pies as “two of the winningest teams among major league bowling leagues.”